1. Field of the Invention
The present invention relates to method of indicating an open/close state of a check valve when driving a screw in a direction of injection in an injection molding machine having a check valve on the screw.
2. Description of the Related Art
Injection molding machines having an injection unit provided with a check valve at a tip of a screw to prevent backflow of resin during injection, such as an in-line screw type injection molding machine, have conventionally been used.
FIG. 1 shows one example of such a check valve. A check ring 3 that is capable of moving in the axial direction of the screw is disposed at a portion of reduced diameter between the screw head 2 mounted on the tip of the screw 1 and the body of the screw 1, and a check seat 4 that contacts and closely adheres to the check ring 3 to close a resin flow channel is provided on a screw 1 body side of the portion of reduced diameter. Resin pellets supplied to the interior of a cylinder 7 from the rear of the screw 1 are melted by shear heat generated by rotation of the screw 1 during metering and by heat from a heater provided on the outside of the cylinder 7 into which the screw 1 is inserted. The melted resin causes the resin pressure behind the check ring 3 to increase, generating a force that pushes the check ring 3 forward. As the check ring 3 is pushed forward, the rearward resin passes through a gap between the check ring 3 and the portion of reduced diameter and flows in front of the check ring 3, increasing the pressure inside the cylinder 7 in front of the screw head 2.
When the pressure in front of the check ring 3 exceeds a predetermined pressure (back pressure), the screw 1 is pushed back and the pressure in front of the check ring 3 is reduced. As the screw 1 rotates further, the pressure behind the check ring 3 becomes higher than the pressure in front of the check ring 3, and the melted resin continues to flow to the front of the check ring 3. When the screw 1 retreats a predetermined amount, screw rotation is stopped to terminate the metering process.
Next is the injection process, in which, as the screw 1 advances to fill a mold with the resin, the resin pressure building ahead of the screw head 2 increases, causing the check ring 3 to retreat and adhere closely to the check seat 4, closing the resin flow channel and preventing the melted resin from flowing backward (back-flowing) in the direction of retreat of the screw 1 due to fill pressure.
During injection the check valve is closed because the pressure in front of the check ring 3 becomes greater than the pressure behind the check ring 3 due to the advance of the screw 1. However, immediately prior to injection, the rear of the check ring 3 is subjected to pressure from resin in a compressed state accumulated in grooves 6 between flights 5, and under the influence of this pressure the timing with which the check valve closes fluctuates, which is a problem. During the time period from the start of injection to the closing of the check valve there occurs a backflow of resin from the front of the check valve toward the rear, and therefore fluctuation in the timing of the closing of the check valve produces fluctuation in injection volume at each cycle, which affects the quality of the molded article.
Consequently, means to enable the check valve to close consistently at the same time in every cycle have been considered, and at the same time methods of monitoring the actual timing with which the check valve closes have been proposed. Moreover, detecting the position of the screw when the check valve closes and measuring the time from the start of injection have been used to set molding conditions, such as the position at which to switch from injection velocity control to pressure holding control and the velocity switchover position, as well as to provide a basis on which to judge the quality of the molded article.
For example, inventions are known that provide a pressure sensor that detects the resin pressure inside the cylinder at a position behind the check valve, detects the closing of the check valve based on a change in pressure detected by such pressure sensor as the screw advances, judge the quality of the articles and adjust the molding conditions based on the position at which the closing of the check valve is detected (see, for example, JP04-53720A and JP04-201225A).
In addition, an arrangement is known that disposes an electrically conductive member behind and opposite a ring valve of a check valve and detects the position of the ring valve, which is to say the time at which the resin flow channel is closed by the ring valve, by measuring the amount of static electricity between the ring valve and the conductive member (see JP03-92321A).
Moreover, in JP01-168421A, an arrangement is described that has nothing that detects the timing of the closing of the check valve during injection but which detects torque exerted on the screw during injection and detects a malfunction of the check valve such as damage or the like by the torque so detected.
Further, an invention is also known that makes use of the fact that, although as the screw is rotated freely and injection starts the resin flows and causes the screw to rotate, once the check valve closes and the backflow of resin stops the rotation of the screw also stops. The invention detects this halting of the rotation of the screw as the time at which the check valve closes, and further, corrects the injection velocity switching position and the position at which the switch is made to pressure holding based on the position at which the closing of the check valve is detected (JP2004-216808A).
However, the inventions described in JP04-53720A and JP04-201225A detect the closing of the check valve by detecting a change in pressure inside the cylinder, and this method requires the addition of a pressure sensor behind the check valve. The pressure sensor must be separated from the front end of the cylinder by at least a distance greater than the maximum injection stroke, and as a result the distance between the check valve and the pressure sensor varies as the injection stroke is larger or smaller, which in turn creates variation in detection accuracy. In addition, although it is preferable that the inner wall surface of the cylinder be without gradations and form a smooth flow channel so that carbide due to resin accumulation does not form, once a pressure sensor that directly contacts the resin is installed the occurrence of tiny gradations in the inner wall surface of the cylinder is unavoidable, with deleterious effects such as carbide due to resin accumulation getting mixed into the molded article. Moreover, with a pressure sensor of a type that indirectly detects the pressure of the resin by detecting deformation of the cylinder indirectly without directly contacting the resin, detection accuracy is sacrificed. Further, pressure sensors of all types are expensive and difficult to handle, and many require periodic maintenance and correction as well.
In addition, detection systems that detect the closing of the ring valve by measuring the amount of static electricity like the invention described in JP03-92321A must add means for measuring the amount of static electricity, such as providing a conductive member for measuring the amount of static electricity on the screw, punching a hole for passing wiring in the center of the screw, and further, providing a slip ring for outputting measurement signals on the screw, which complicates the configuration.
The invention described in JP2004-216808A focuses on a component force Fθ in a direction of rotation of the screw of a force exerted on the flights by backflow resin, and as noted above detects the closing of the check valve by detecting a halt in the rotation of the screw, which rotates freely during injection.
However, when the rotary screw is rotated by the backflow resin, as long as the backflow amount is small, the force exerted by the backflow resin on the screw to make the screw rotate is within a maximum static frictional force range of the screw against the cylinder, and therefore the screw does not rotate. Then, as the backflow amount increases and the force exerted on the screw causing the screw to rotate exceeds the maximum static frictional force, the screw begins to rotate. Once the screw begins to rotate there is a shift to a dynamic friction range, and therefore even when the force exerted by the backflow resin on the screw causing the screw to rotate falls below the maximum static frictional force the screw nevertheless continues to rotate if the force exerted on the screw causing the screw to rotate by the backflow resin is equal to or greater than the dynamic frictional force. As a result, when the force exerted on the screw by the backflow resin causing the screw to rotate is equal to or greater than the dynamic frictional force but equal to or less than the maximum static frictional force, if the rotation of the screw has already halted then that stopped state continues; if the screw has already been rotating, then that state of rotation continues. Thus, it cannot be said that there is always a linear relation between the size of the backflow amount and the screw rotation amount, and as a result, in a method of detecting the closing of the check valve from the screw rotation amount as in the invention described in JP2004-216808A, there is the possibility of including error in the detection of the valve closing timing.
Moreover, with the invention described in JP2004-216808A, although it detects the halting of the rotation of the screw, in order to detect that halt in rotation it is necessary to set some sort of threshold value. Sometimes the screw stops rotating slowly and sometimes quickly, and therefore, in order to detect accurately a halt in rotation under such varying conditions, it is necessary to set the threshold value appropriately. However, obtaining the appropriate threshold value requires time and effort, and moreover, the threshold value must be readjusted every time there is a change in the conditions of molding.